ArmPlatformPkg/PrePi RVCT: use relative reference to mSystemMemoryEnd

ArmPlatformPkg/PrePi RVCT: use relative reference to mSystemMemoryEnd

Bring the RVCT version of the PrePi entry point code in line with the
GCC version, by using a relative rather than an absolute reference to
the mSystemMemoryEnd variable.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Leif Lindholm <leif.lindholm@linaro.org>